Not Applicable
1. Field of the Invention
The invention relates to emergency lighting, and particularly to fluorescent lighting wherein a ballast for a fluorescent lamp is connected to a source of electrical energy other than normal AC line current in the event that the normal AC current fails.
Emergency lighting is required in commercial, industrial, and institutional buildings just as fire extinguishers, smoke alarms and other safety equipment. Three types of emergency lighting are common in such installations: unit equipment, engine generators and central battery systems. Unit equipment falls into two principle types: fluorescent and incandescent.
The fluorescent units are customarily combined with and within a conventional fluorescent lighting unit by merely adding the emergency ballast consisting of a battery, a battery charger, inverter and sensing circuitry adjacent the standard fluorescent AC ballast. The sensing circuit of the emergency ballast observes the interruption of normal AC power to the lamp unit and immediately switches on the emergency ballast to power individual lamp(s) or the light fixture for the required period which, under most state safety codes, is a period of at least ninety (90) minutes, a standard called out in the National Electrical Code, NFPA Article 70, and NFPA Article 101 Light Safety Code.
2. General Background of the Invention
U.S. Pat. No. 5,004,953 entitled Emergency Lighting Ballast for Compact Fluorescent Lamps with Integral Starters, assigned to the assignee of the present invention is illustrative of the general fluorescent type of emergency lighting with a ballast. It is common in the installation of emergency fluorescent lighting that an emergency ballast is added to a conventional fluorescent fixture or provided integrally in a fixture having internal regular and emergency ballast installed. When main AC power fails, voltage sensing circuitry instantly connects DC current from a battery (in the emergency ballast) to an inverter which produces high frequency, high voltage power to illuminate the emergency fluorescent lamp(s) for the required period.
The present invention is directed to fluorescent lighting fixtures which incorporate small fluorescent lamps, such as those which have a smaller diameter than conventional fluorescent bulbs (e.g. about ⅝xe2x80x3). These lamps are coming into more common usage and are employed in single or multiple lamp, low profile fixtures. In such small diameter lamps, the cathodes at the lamp ends are very close to the glass envelope. When this type of fluorescent lamp approaches end of its normal life, high power is generated in the cathodes, which may get very hot and can crack the glass open adjacent the cathode heaters. Standard ballasts would continue to supply high voltage to the cracked lamp, which would create potentially dangerous exposure to laceration if someone would try to unknowingly replace cracked glass lamps, as by causing further cracking or open breakage of the glass envelope and impingement of the sharp edges into the skin. The continued operation of the AC ballast with the damaged (unilluminated) lamp may also create an electrical shock hazard were the glass to disintegrate and allow an individual to touch the xe2x80x9chotxe2x80x9d cathode (i.e., one carrying high voltage).
To prevent this electrical shock hazard, the electronic AC ballasts for small fluorescent lamps now include an end-of-lamp-life shut down circuit. These A.C. ballasts now incorporate a circuit to sense the increased cathode voltage and shut the high voltage down that normally would be supplied to the cracked lamp. Manufacturers that sell ballasts incorporating such a feature are Energy Savings Inc. (Lamp Guard, or Super Lamp Guard), Osram Quicktronic, and Magnetek.
These new electronic shutdown circuits conventionally sense any sudden change in power supplied to the lamp, such as a sudden increase in AC voltage or any DC voltage developed across the lamp. If any of these conditions is detected, the AC ballast operation is shut down such that no high voltage appears at the lamp.
This shut down capability frequently interferes with the inclusion of an emergency ballast which otherwise will operate the lamp in the event of AC power failure. The problem is actually created with the restoration of normal AC power after the lamp has been powered by the emergency ballast as a result of A.C. power failure. With the shift from battery operation of the lamp by the emergency ballast, there are transient swings of voltage and power to the lamp as the emergency ballast output shuts down and the AC ballast resumes operation. These transient voltages frequently trigger the shut down circuit since the transients exhibit symptoms similar to the voltage swings of the small lamp reaching its end-of-life state. The present invention coordinates the restarting of the fluorescent lamps with normal AC power with the cessation of the supply of emergency power from the emergency ballast.
The present invention is directed to a low profile emergency ballast for operation in conjunction with a low profile AC ballast having an end of lamp life shut down circuit. More particularly, the present invention is directed to an emergency ballast for a fluorescent lamp having means to avoid erroneous action of an end of lamp life shut down circuit in a low profile AC ballast.
One of the objectives of the present invention is the momentary delay of the powering of the lamp by the resumed A.C. power in order to allow the transients exhibited by the shut down of the emergency ballast to subside. A further objective of the present invention is the inclusion of a delay circuit which operates only on A.C. power application directly following operation of the emergency ballast.